Low attenuation fiber with viscosity matched core and inner clad

ABSTRACT

A single mode optical fiber having a core made from silica and less than or equal to about 6.5 weight % germania and having a maximum relative refractive index Δ 1MAX . The optical fiber also has an inner cladding surrounding the core and having a minimum relative refractive index Δ 2MIN . A difference between a softening point of the core and a softening point of the inner cladding is less than or equal to about 20° C., and Δ 1MAX &gt;Δ 2MIN . The single mode optical fiber may also have an outer cladding surrounding the inner cladding made from silica or SiON. The outer cladding has a maximum relative refractive index Δ 3MAX , and Δ 3MAX &gt;Δ 2MIN . A method for manufacturing an optical fiber includes providing a preform to a first furnace, the preform, drawing the optical fiber from the preform, and cooling the drawn optical fiber in a second furnace.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority under 35 U.S.C. §119 ofU.S. Provisional Application Ser. No. 62/016,192 filed on Jun. 24, 2014the contents of which are relied upon and incorporated herein byreference in their entirety.

BACKGROUND

Field

The present disclosure relates generally to optical fibers, andparticularly to low attenuation optical fibers.

Technical Background

Glass optical fibers with low attenuation have recently been ofsignificant interest in the telecommunications field. Techniques forimproving attenuation properties can play important roles in many typesof fibers, including transmission fibers used in long distanceapplications, multimode fibers used in the emerging area of fiber to thehome applications, and dispersion compensation fibers where bending losshas limited many designs from practical use. In certain applicationssuch as long distance applications, low attenuation is desired todeliver data accurately via light signals. Many of the proposedsolutions for this problem involve significant modification of the fiberand its refractive index profile.

SUMMARY

According to one or more embodiments shown and described herein a singlemode optical fiber has a core made from silica and less than or equal toabout 6.5 weight % germania and has a maximum relative refractive indexΔ_(1MAX). The optical fiber also has an inner cladding surrounding thecore and having a minimum relative refractive index Δ_(2MIN). Adifference between a softening point of the core and a softening pointof the inner cladding is less than or equal to about 20° C., andΔ_(1MAX)>Δ_(2MIN).

According to some embodiments shown and described herein, the singlemode optical fiber may also have an outer cladding surrounding the innercladding made from silica or SiON. The outer cladding has a maximumrelative refractive index Δ_(3MAX), and Δ_(3MAX)>Δ_(2MIN).

According to embodiments shown and described herein a method ofmanufacturing a single mode optical fiber includes providing a preformto a first furnace, the preform having a core comprising silica and lessthan or equal to about 6.5 weight % germania, and an inner cladding thatsurrounds the core; drawing the optical fiber from the preform, andcooling the drawn optical fiber in a second furnace, wherein adifference between a softening point of the core and a softening pointof the inner cladding is less than or equal to about 50° C.

Additional features and advantages of embodiments will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing embodiments as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments, and are intendedto provide an overview or framework for understanding the nature andcharacter of embodiments as they are claimed. The accompanying drawingsare included to provide a further understanding of embodiments, and areincorporated into and constitute a part of this specification. Thedrawings illustrate various embodiments, and together with thedescription serve to explain the principles and operations ofembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross sectional view of an optical fiberaccording to one or more embodiments shown and described herein;

FIG. 1B graphically depicts index versus radius of an optical fiberdepicted in FIG. 1A;

FIG. 2 is a schematic of a system for drawing an optical fiber accordingto one or more embodiments shown and described herein;

FIG. 3 graphically depicts Δ % versus radius of optical fibers accordingto one or more embodiments shown and described herein; and

FIG. 4 graphically depicts axial stress versus radius of optical fibersaccording to one or more embodiments shown and described herein.

DETAILED DESCRIPTION

FIG. 1A schematically depicts a cross section of an optical fiber 100according to one or more embodiments shown and described herein.Embodiments of optical fibers 100 described herein generally comprise asingle mode optical fiber having a core 102 made from silica and lessthan or equal to about 6.5 weight % germania. FIG. 1B graphicallydepicts an index profile versus radius of the fiber 100 depicted in FIG.1A. The core 102 has a maximum relative refractive index Δ_(1MAX). Theoptical fiber 100 also has an inner cladding 104 surrounding the core102 and having a minimum relative refractive index Δ_(2MIN). Adifference between a softening point of the core 102 and a softeningpoint of the inner cladding 104 is less than or equal to about 20° C.,and Δ_(1MAX)>Δ_(2MIN). In some embodiments, the optical fiber 100 alsohas an outer cladding 106 surrounding the inner cladding 104.

The “refractive index profile,” as used herein, is the relationshipbetween refractive index or relative refractive index and fiber radiusof a radial cross section of the optical fiber.

“Relative refractive index,” as used herein, is defined as:

${\Delta_{i}\mspace{14mu}\%} = {100 \times \frac{\left( {n_{i}^{2} - n_{ref}^{2}} \right)}{2\; n_{i}^{2}}}$where n_(i) is the maximum refractive index in region i, unlessotherwise specified, and n_(ref) is the refractive index of pure silicaglass, unless otherwise specified. Accordingly, as used herein, therelative refractive index percent is relative to pure silica glass. Theterms delta, delta index, delta index percent, Δ, Δ % are usedinterchangeably herein.

More specifically, as used herein, Δ_(1MAX) refers to the maximumrelative refractive index of a core of the optical fiber, Δ_(2MIN)refers to the minimum relative refractive index of an inner cladding ofthe optical fiber and Δ_(3MAX) refers to the maximum relative refractiveindex of an outer cladding of the optical fiber. The relative refractiveindexes are given in percentages based from the refractive index of puresilica glass.

It should be understood that the phrase “pure silica glass,” as usedherein, means that the region or layer of the optical fiber comprising“pure silica glass” does not contain material, such as dopants and/orother trace materials, in an amount which would significantly alter therefractive index of the silica glass region or portion. However, smallamounts of dopants (e.g., chlorine and/or fluorine in an amount lessthan 1500 ppm of each) may be present in the region or portion that isotherwise “pure silica.”

“Chromatic dispersion” (which may be referred to herein as “dispersion”unless otherwise noted) of a waveguide fiber is the sum of the materialdispersion and the waveguide dispersion. A zero dispersion wavelength isa wavelength at which the dispersion has a value of zero and alsoreferred to herein as Lambda 0 or 4. Dispersion slope is the rate ofchange of dispersion with respect to wavelength.

“Effective area” is defined in equation 1 as:A _(eff)=2π(∫f ² rdr)²/(∫f ⁴ rdr)  (Eq. 1)

where the integration limits are 0 to ∞, and f is the transversecomponent of the electric field associated with light propagated in thewaveguide. As used herein, “effective area” or “Δ_(eff)” refers tooptical effective area at a wavelength of 1550 nm unless otherwisenoted.

The term “α-profile” (also referred to herein as alpha profile or justalpha) refers to a relative refractive index profile of the core regionexpressed in terms of Δ(r) which is in units of “%”, where r is radius.Δr is represented by equation 2,Δ(r)=Δ(r _(o))(1−[|r−r _(o)|/(r ₁ −r _(o))]^(α))  (Eq. 2)

where r_(o) is the point at which Δ(r) is maximum, r₁ is the point atwhich Δ(r) is zero, and r is in the range r_(i)<r<r_(f), where Δ isdefined above, r_(i) is the initial point of the α-profile, r_(f) is thefinal point of the α-profile, and α is an exponent which is a realnumber.

The mode field diameter (MFD) is measured using the Peterman II methodas shown in equations 3 and 4, respectively wherein,2w=MFD  (Eq. 3)andw ²=(2πf ² rdr/∫[df/dr] ² rdr)  (Eq. 4)

wherein the integral limits are 0 to ∞.

The bend resistance of a waveguide fiber can be gauged by inducedattenuation under prescribed test conditions, such as by deploying orwrapping the fiber around a mandrel having a prescribed diameter, e.g.,by wrapping 1 turn around either a 6 mm, 10 mm, 20 mm, 30 mm or similardiameter mandrel (e.g. “1×10 mm diameter macrobend loss” or the “1×30 mmdiameter macrobend loss”) and measuring the increase in attenuation perturn.

One type of bend test is the lateral load microbend test. In a so-called“lateral load wire mesh” test (LLWM), a prescribed length of waveguidefiber is placed between two flat plates. A #70 wire mesh is attached toone of the plates. A known length of waveguide fiber is sandwichedbetween the plates, and a reference attenuation is measured while theplates are pressed together with a force of 30 Newtons. A 70 Newtonforce is then applied to the plates and the increase in attenuation indB/m is measured. The increase in attenuation is the lateral loadattenuation of the waveguide in dB/m at a specified wavelength(typically within the range of 1200-1700 nm, e.g., 1310 nm or 1550 nm or1625 nm).

The “pin array” bend test is used to compare relative resistance ofwaveguide fiber to bending. To perform this test, attenuation loss ismeasured for a waveguide fiber with essentially no induced bending loss.The waveguide fiber is then woven about the pin array and attenuationagain measured. The loss induced by bending is the difference betweenthe two measured attenuations. In embodiments, the pin array is a set often cylindrical pins arranged in a single row and held in a fixedvertical position on a flat surface. The pin spacing is 5 mm, center tocenter, and the pin diameter is 0.67 mm. During testing, sufficienttension is applied to make the waveguide fiber conform to a portion ofthe pin surface. The increase in attenuation is the pin arrayattenuation in dB of the waveguide at a specified wavelength (typicallywithin the range of 1200-1700 nm, e.g., 1310 nm or 1550 nm or 1625 nm).

The theoretical fiber cutoff wavelength, “theoretical fiber cutoff”, or“theoretical cutoff” for a given mode is the wavelength above whichguided light cannot propagate in that mode. A mathematical definitioncan be found in “Single Mode Fiber Optics,” Jeunhomme, pp. 39-44, MarcelDekker, New York, 1990 wherein the theoretical fiber cutoff is describedas the wavelength at which the mode propagation constant becomes equalto the plane wave propagation constant in the outer cladding. Thistheoretical wavelength is appropriate for an infinitely long, perfectlystraight fiber that has no diameter variations.

Fiber cutoff is measured by the standard 2 m fiber cutoff test, FOTP-80(EIA-TIA-455-80), to yield the “fiber cutoff wavelength,” also known asthe “2 m fiber cutoff” or “measured cutoff.” The FOTP-80 standard testis performed to either strip out the higher order modes using acontrolled amount of bending, or to normalize the spectral response ofthe fiber to that of a multimode fiber.

By cabled cutoff wavelength, or “cabled cutoff” as used herein, we meanthe 22 m cabled cutoff test described in the EIA-445 Fiber Optic TestProcedures, which are part of the EIA-TIA Fiber Optics Standards, thatis, the Electronics Industry Alliance—Telecommunications IndustryAssociation Fiber Optics Standards.

Unless otherwise noted herein, optical properties (such as dispersion,dispersion slope, etc.) are reported for the LP01 mode.

Referring to FIGS. 1A and 1B, a cross section of an optical fiber 100 isshown according to embodiments described herein. The optical fiber 100generally comprises a glass core 102 with an inner cladding 104surrounding the core 102. In some embodiments, an outer cladding 106 maysurround the inner cladding 104. The core 102, the inner cladding 104,and the outer cladding 106 may comprise silica, specificallysilica-based glass. The core 102 and the inner cladding 104 may comprisedopants, as described in more detail herein. The cross section of theoptical fiber 100 may be generally circular-symmetric with respect tothe center of the core 102 and the core 102 may have a radius r₁. Theinner cladding 104 surrounds the core 102 and extends from the radius r₁to the radius r₂ such that the inner cladding has a radial thicknessT₂=r₂−r₁. The outer cladding 106 surrounds the inner cladding 104 andextends from the radius r₂ to the radius r₃ such that the outer claddinghas a radial thickness T₃=r₃−r₂. Accordingly, the optical fiber 100(e.g., the core 102, inner cladding 104 and outer cladding 106) may havean outer diameter 2r₃.

As described herein, the core 102 of the optical fiber 100 has a radiusr₁ and a radial thickness T₁=r₁. In embodiments, the optical fiber 100may be a single-mode optical fiber. The core may have a radial thicknessof greater than or equal to about 3.0 microns, such as greater than orequal to about 4.0 microns. The core may have a radial thickness lessthan or equal to about 7.0 microns, such as less than or equal to about6.0 microns. Accordingly, in some embodiments, the radial thickness T₁may be from greater than or equal to about 3.0 microns to less than orequal to about 7.0 microns, such as from greater than or equal to about4.0 microns to less than or equal to about 6.0 microns. In otherembodiments, the radial thickness T₁ may be about 5.0 microns. However,it should be understood that the core 102 may have different dimensionsto facilitate various other single-mode embodiments.

In embodiments, the core 102 comprises silica glass (SiO₂) and one ormore index of refraction raising dopants (referred to hereinafter as “updopants”) such as, for example, GeO₂, Al₂O₃, P₂O₅, TiO₂, ZrO₂, Nb₂O₅and/or Ta₂O₅. Without being bound to any particular theory, it isbelieved that dopants, such as GeO₂, in the core 102 of the opticalfiber 100 cause Rayleigh scattering of light conducted within the core102 of the optical fiber 100, causing attenuation along the length ofthe optical fiber. Optical fibers having higher concentrations ofdopants will generally have more Rayleigh scattering, which leads toincreased attenuation. Accordingly, embodiments of the optical fiberdescribed herein have low core dopant concentrations, which improve theattenuation properties of the optical fiber.

In some embodiments, the core 102 is up-doped with GeO₂. For example,the core 102 may be up-doped with less than or equal to about 6.5 weight% GeO₂, such as less than or equal to about 6.0 weight % GeO₂. The core102 may be up-doped with less than or equal to about 5.5 weight % GeO₂,such as less than or equal to about 5.0 weight % GeO₂. In embodiments,the core 102 may be up-doped with greater than or equal to about 2.0weight % GeO₂, such as greater than or equal to about 2.5 weight % GeO₂.In embodiments, the core 102 may be up-doped with greater than or equalto about 3.0 weight % GeO₂, such as greater than or equal to about 3.5weight % GeO₂. Accordingly, in embodiments, the core 102 may comprisefrom greater than or equal to about 2.0 weight % to less than or equalto about 6.5 weight % GeO₂, or from greater than or equal to about 2.5weight % to less than or equal to about 6.0 weight % GeO₂. The core 102may comprise from greater than or equal to about 3.0 weight % to lessthan or equal to about 5.5 weight % GeO₂, or from greater than or equalto about 3.5 weight % to less than or equal to about 5.0 weight % GeO₂.

In embodiments where the core 102 is up-doped, the maximum relativerefractive index Δ_(1MAX) of the core 102 may be greater than or equalto about 0.13%, such as greater than or equal to about 0.15%. Inembodiments, the maximum relative refractive index Δ_(1MAX) may begreater than or equal to about 0.20%, such as greater than or equal toabout 0.23%. The maximum relative refractive index Δ_(1MAX) may be lessthan or equal to about 0.37%, such as less than or equal to about 0.35%.In embodiments, the maximum relative refractive index Δ_(1MAX) may beless than or equal to about 0.30%, such as less than or equal to about0.27%. Accordingly, in embodiments, the maximum relative refractiveindex Δ_(1MAX) may be from greater than or equal to about 0.13% to lessthan or equal to about 0.37%, such as from greater than or equal toabout 0.15% to less than or equal to about 0.35. The maximum relativerefractive index Δ_(1MAX) of the core 102 may be from greater than orequal to about 0.20% to less than or equal to about 0.30%, such as fromgreater than or equal to about 0.23% to less than or equal to about0.27%.

As described herein above, the optical fiber 100 may further comprise aninner cladding 104. In embodiments, the inner cladding 104 has a radialthickness T₂=r₂−r₁. The radial thickness T₂ of the inner cladding 104may depend on the desired dimensions of the core 102 and the desireddimensions and properties of the glass portion of the optical fiber 100.In embodiments, the inner cladding may have a radial thickness ofgreater than or equal to about 12.0 microns, such as greater than orequal to about 25.0 microns. The inner cladding may have a radialthickness of greater than or equal to about 30.0 microns, such asgreater than or equal to about 35.0 microns. The inner cladding may havea radial thickness of less than or equal to about 55.0 microns.Accordingly, in embodiments, the inner cladding may have a radialthickness from greater than or equal to about 12.0 microns to less thanor equal to about 55.0 microns, such as from greater than or equal toabout 25.0 microns to less than or equal to about 55.0 microns. Theinner cladding may have a radial thickness from greater than or equal toabout 30.0 microns to less than or equal to about 55.0 microns, such asfrom greater than or equal to about 35.0 microns to less than or equalto about 55.0 microns.

In embodiments, the inner cladding may comprise silica-based glass anddopants that decrease the refractive index of the inner cladding(hereinafter referred to as “down dopants”), such as fluorine. The innercladding may be down-doped with greater than or equal to about 0.10weight % fluorine, such as greater than or equal to about 0.12 weight %fluorine. In embodiments, the inner cladding may be down-doped withgreater than or equal to about 0.20 weight % fluorine, such as greaterthan or equal to about 0.30 weight % fluorine. The inner cladding may bedown-doped with less than or equal to about 0.65 weight % fluorine, suchas less than or equal to about 0.50 weight % fluorine. In embodiments,the inner cladding may be down-doped with less than or equal to about0.45 weight % fluorine, such as less than or equal to about 0.40 weight% fluorine. Accordingly, in embodiments, the inner cladding may bedown-doped with from greater than or equal to about 0.10 weight %fluorine to less than or equal to about 0.65 weight % fluorine, such asfrom greater than or equal to about 0.12 weight % fluorine to less thanor equal to about 0.50 weight % fluorine. The inner cladding may bedown-doped with from greater than or equal to about 0.20 weight %fluorine to less than or equal to about 0.45 weight % fluorine, such asfrom greater than or equal to about 0.30 weight % fluorine to less thanor equal to about 0.40 weight % fluorine.

In embodiments, the inner cladding has a minimum relative refractiveindex Δ_(2MIN) that is less than the relative refractive index of puresilica glass. For example, the inner cladding may have a minimumrelative reflective index Δ_(2MIN) of less than or equal to about−0.040%, such as less than or equal to about −0.050%. The inner claddingmay have a minimum relative reflective index Δ_(2MIN) of less than orequal to about −0.100%, such as less than or equal to about −0.125%. Theinner cladding may have a minimum relative refractive index Δ_(2MIN)greater than or equal to about −0.210%, such as greater than or equal toabout −0.200%. The inner cladding may have a minimum relative refractiveindex Δ_(2MIN) greater than or equal to about −0.175%, such as greaterthan or equal to about −0.150%. Accordingly, in embodiments, the innercladding may have a minimum relative refractive index Δ_(2MIN) from lessthan or equal to about −0.040% to greater than or equal to about−0.210%, such as from less than or equal to about −0.050% to greaterthan or equal to about −0.200%. The inner cladding may have a minimumrelative refractive index Δ_(2MIN) from less than or equal to about−0.100% to greater than or equal to about −0.175%, such as from lessthan or equal to about −0.125% to greater than or equal to about−0.150%.

Doping the core 102 with an up dopant and doping the inner cladding 104with a down dopant provides a relationship between the maximum relativerefractive index of the core Δ_(1MAX) and the minimum relativereflective index of the inner cladding Δ_(2MIN) where Δ_(1MAX)>Δ_(2MIN).Further, the concentration of up dopants in the core 102 and theconcentration of down dopants in the inner cladding 104 may be used tomatch the viscosity at the core/inner cladding boundary. Matching theviscosity of the core 102 and the viscosity of the inner cladding 104reduces interfacial fluctuations at the boundary of the core 102 withthe cladding 104, which cause small angle scattering of light within theoptical fiber, further increasing attenuation of light traveling withinthe core 102 of the optical fiber 100. The viscosities of the core andthe cladding in conventional fibers are generally not matched. Themismatch of these viscosities in conventional fibers leads todisplacement of the core to the inner cladding at a given temperature,such as temperatures between the softening points of the core and theinner cladding. The displacement causes interfacial fluctuations thatbecome permanent when the glass is cooled and, thereby, increasesattenuation.

The matching of viscosity of the core 102 and the viscosity of the innercladding 104 may be assessed by the difference between the softeningpoint of the core 102 and the softening point of the inner cladding 104.As should be understood, the softening point of the core 102 and theinner cladding 104 is the temperature where the composition has aviscosity of 10^(7.6) poise. In embodiments, a difference between thesoftening point of the core and a softening point of the inner claddingmay be less than or equal to about 15° C., such as less than or equal toabout 12° C. A difference between the softening point of the core and asoftening point of the inner cladding may be less than or equal to about10° C., such as less than or equal to about 8° C.

As described herein above, and shown in FIGS. 1A and 1B, the opticalfiber 100 may comprise an outer cladding 106. The outer cladding 106 hasa radial thickness T₃=r₃−r₂. In embodiments, the radial thickness T₃ ofthe outer cladding 106 may be less than or equal to about 47.5 microns,such as less than or equal to about 34.5 microns. In other embodiments,the radial thickness T₃ of the outer cladding 106 may be less than orequal to about 29.5 microns, such as less than or equal to about 24.5microns. In some embodiments, the outer cladding 106 is optional.

In embodiments, the outer cladding 106 comprises pure silica glass orSiON.

Therefore, the maximum relative refractive index Δ_(3MAX) of the outercladding is about 0.0% because, as stated herein, the relativerefractive index is based on the refractive index of pure silica glass.Additionally, the outer cladding 106 will be stiff compared to the core102 and the inner cladding 104, because it is not doped. Although notbeing bound to any particular theory, the outer cladding 106 contributesto reducing attenuation by reducing thermal stresses caused bymismatched CTE in the core 102 and the inner cladding 104. A mismatchedCTE in the core 102 and the inner cladding can cause one of the core 102or the cladding 104 to expand or contract more than the other, therebycausing stresses in core 102 and/or inner cladding 104 that can resultin fluctuations that increase attenuation. However, when the outercladding is stiffer than the core 102 and the inner cladding 104, itwill expand or contract less than the core 102 or the inner cladding104. Thereby, the stresses caused by mismatched CTE between the core 102and the cladding 104 are transferred to the outer cladding 106, andfluctuations are reduced in the core 102 and the inner cladding 104.Accordingly, in some embodiments, the outer cladding 106 is positionedso that it does not interfere with the light that travels through theoptical fiber 100.

As described herein above, according to embodiments, the relativerefractive indexes of the core 102, the inner cladding 104, and theouter cladding 106 satisfy the following relationship:Δ_(1MAX)>Δ_(3MAX)>Δ_(2MIN).

Embodiments of the optical fiber disclosed herein have reducedattenuation. For example, the optical fiber may have an attenuation ofless than or equal to about 0.19 dB/km at a wavelength of 1550 nm. Insome embodiments, the optical fiber may have an attenuation of less thanor equal to about 0.18 dB/km at a wavelength of 1550 nm, such as lessthan or equal to about 0.175 dB/km at a wavelength of 1550 nm. Theoptical fiber may have an attenuation of less than or equal to about0.17 dB/km at a wavelength of 1550 nm, such as less than or equal toabout 0.165 dB/km at a wavelength of 1550 nm. Additionally, the opticalfiber may have an attenuation of less than or equal to about 0.32 dB/kmat a wavelength of 1310 nm, such as less than or equal to about 0.31dB/km at a wavelength of 1310 nm. The fiber designs disclosed hereinresult in fibers having optical properties that are G.652 compliant(ITU-T standards), MFD from greater than or equal to about 8.2 to lessthan or equal to about 9.5 microns at 1310 nm, such as from greater thanor equal to about 9.0 to less than or equal to about 9.4 microns at 1310nm, zero dispersion wavelength, λ₀, of 1300≦λ₀≦1324 nm, and cable cutoffless than or equal to about 1260 nm. Additional fiber designs disclosedherein result in fibers having optical properties that are G.654compliant (ITU-T standards), and for example may exhibit a cable cutoffless than 1530 nm, such as less than 1500 nm. The G.654 applications thefibers may be configured to have dispersion at 1550 nm, which is lessthan or equal to 22 ps/nm/km.

In embodiments, the optical fiber may be a large effective area opticalfiber. For example, the optical fiber may have an effective area greaterthan or equal to about 70 microns² at a wavelength of 1550 nm, such asgreater than or equal to about 80 microns² at a wavelength of 1550 nm.The optical fiber may have an effective area greater than or equal toabout 90 microns² at a wavelength of 1550 nm, such as greater than orequal to about 100 microns² at a wavelength of 1550 nm. The opticalfiber may have an effective area less than or equal to about 145microns² at a wavelength of 1550 nm, such as less than or equal to about135 microns² at a wavelength of 1550 nm. The optical fiber may have aneffective area less than or equal to about 125 microns² at a wavelengthof 1550 nm, such as less than or equal to about 155 microns² at awavelength of 1550 nm. Accordingly, in embodiments, the optical fibermay have an effective area of from greater than or equal to about 70microns² to less than or equal to about 145 microns², such as fromgreater than or equal to about 80 microns² to less than or equal toabout 135 microns². The optical fiber may have an effective area of fromgreater than or equal to about 90 microns² to less than or equal toabout 125 microns², such as from greater than or equal to about 100microns² to less than or equal to about 115 microns².

In embodiments, the core 102, inner cladding 104, and outer cladding 106of the optical fiber 100 may be formed by an outside-vapor-deposition(OVD) process. The OVD process is a way of making optical fiber throughreactions from the desired vapor ingredients (including silica and theother desired up dopant precursors) via a hydrolysis process in a CH₄+O₂flame to form soot-particles (such as in the range of about 2 nm to 5microns in diameter, and in some embodiments in the range of about 50 to500 nm in diameter). The soot-particles are then collected bythermopheretic means onto either a bait rod (for making a coresoot-preform) or a glass core cane or rod (for making overcladsoot-preform). The soot-preform is subsequently dried and densified intosolid transparent glass in a high temperature furnace (after the baitrod is removed from the core preform), a process commonly referred to asconsolidation. The desired core and cladding compositions are achievedby utilizing different amounts of various vapor-phase ingredients foreach of the layers in the soot preform fabrication process. For example,the core/inner cladding/outer cladding preform may be generated first,then consolidated, and the final (consolidated) preform drawn into theoptical fiber 100 by known fiber-drawing methods.

More specifically, vapor-precursor-materials that may be used to makethe portion of the soot preform associated with the fiber core are, inembodiments, SiCl₄, GeCl₄, AlCl₃, TiCl₄, or POCl₃. As described inembodiments herein, the core may comprise GeO₂ doped silica glass. Thissoot preform is placed into a furnace, dried (e.g., in an atmospherecomprising chlorine gas) and then the up-doped SiO₂ soot is consolidatedinto a core preform (also referred to herein as a core glass preform orvoid-free glass core preform). The consolidated core preform is thenoptionally placed and heated in an air-, nitrogen-, or argon-purgedfurnace at about 800-1200° C. to outgas helium dissolved in the glass,and then optionally placed into another furnace and redrawn into one ormultiple canes (also referred to as core canes). Soot of pure SiO₂ isdeposited on the core preform to form a soot preform having a solidglass core cane. This soot/cane assembly is then placed in a furnace,dried, and then doped with fluorine (e.g., in an atmosphere comprisingSiF₄). The assembly is thereafter consolidated to fully densified glass.The consolidated preform is then optionally placed and heated in anair-, nitrogen-, or argon-purged furnace at about 800-1200° C. to outgasthe helium dissolved in the glass, and then optionally placed intoanother furnace and redrawn into one or multiple canes having a GeO₂doped core surrounded by an F-doped silica cladding. The processes ofdepositing additional soot onto the consolidated preform, drying,doping, and sintering to fully densified glass may be repeated. Inembodiments, the preform comprises silica with a GeO₂ doped core/F-dopedinner clad and an F-doped, undoped SiO₂, or SiON doped outer cladding.The consolidated preform is then optionally placed and heated in anair-, nitrogen-, or argon-purged furnace at about 800-1200° C. to outgashelium dissolved in the glass. Optical fiber is then drawn from thepreform and coated with standard primary and secondary urethane acrylatecoatings.

Referring now to FIG. 2, one embodiment of a system 200 for producing anoptical fiber is illustrated. The system 200 may comprise a draw furnace202 for heating an optical fiber preform 204 such that an optical fiber100 may be drawn from the optical fiber preform 204. The preform 204 maybe produced by the OVD method and have the composition and structure asset forth above. The draw furnace 202 may be oriented such that anoptical fiber 100 drawn from the optical fiber preform 204 exits thefurnace along a substantially vertical pathway.

After the optical fiber 100 exits the draw furnace 202, the diameter ofthe optical fiber 100 and the draw tension applied to the optical fiber100 may be measured using non-contact sensors 206 a, 206 b. Tension maybe applied to the optical fiber by any suitable tension-applyingmechanism 210. As shown in FIG. 2, after the diameter and tension of theoptical fiber 100 are measured, the optical fiber 100 may be passedthrough a cooling mechanism 208 which provides slow cooling of theoptical fiber 100. The cooling mechanism 208 may be any mechanism forcooling an optical fiber as may be presently known in the art orsubsequently developed. In one embodiment, the cooling mechanism 208 isfilled with a gas that facilitates cooling of the optical fiber 100 at arate slower than cooling the optical fiber 100 in air at ambienttemperatures. As discussed above, optical fibers of embodiments includelower concentrations of up dopants, such as GeO₂, which means that theglass has a higher concentration of silica. Pure silica glass is stifferthan silica glass comprising up dopants, and the higher theconcentration of up dopants, the lower the stiffness of the silica-basedglass. Accordingly, cores according to embodiments, which have a lowerconcentration of up dopants than cores of conventional optical fibers,are stiffer than cores in conventional optical fibers. This increasedstiffness results in stresses as the optical fiber is cooled because theoptical fiber does not sufficiently contract to absorb the stresses. Thefaster the optical fiber is cooled, the more stresses are introducedinto the optical fiber because the optical fiber cannot contract fastenough. These stresses result in fluctuations that increase attenuation.Accordingly, according to embodiments, the optical fiber may be cooledslowly to allow the stresses to relax and, thereby, reduce fluctuationsand attenuation in the optical fiber.

In embodiments, the cooling mechanism 208 may cool the drawn opticalfiber from a temperature of about 1600° C. to a temperature of about1250° C. at a cooling rate of less than or equal to about 5000° C./s,such as less than or equal to about 4750° C./s. In some embodiments, thecooling mechanism 208 may cool the drawn optical fiber from atemperature of about 1600° C. to a temperature of about 1250° C. at acooling rate of less than or equal to about 4500° C./s, such as lessthan or equal to about 4250° C./s. In some embodiments, the coolingmechanism 208 may cool the drawn optical fiber from a temperature ofabout 1250° C. to a temperature of about 1050° C. at a cooling rate ofless than or equal to about 12000° C./s, such as less than or equal toabout 11500° C./s. The cooling mechanism 208 may cool the drawn opticalfiber from a temperature of about 1250° C. to a temperature of about1050° C. at a cooling rate of less than or equal to about 11000° C./s,such as less than or equal to about 10500° C./s. In some embodiments thecooling mechanism 208 cools the drawn optical fiber from a temperatureof about 1400° C. to a temperature of about 1050° C. at a cooling rateof less than or equal to about 4500° C./s, such as less than or equal toabout 4250° C./s. In some embodiments, the cooling mechanism 208 maycool the drawn optical fiber from a temperature of about 1050° C. to atemperature of about 850° C. at a cooling rate of less than or equal toabout 12000° C./s, such as less than or equal to about 11500° C./s. Thecooling mechanism 208 may cool the drawn optical fiber from atemperature of about 1050° C. to a temperature of about 850° C. at acooling rate of less than or equal to about 11000° C./s, such as lessthan or equal to about 10500° C./s.

In embodiments, the tension-applying mechanism 210 may apply a tensionto the optical fiber 100 of less than or equal to about 100 g_(f),(g_(f) refers to grams force, herein) such as less than or equal toabout 95 g_(f). The tension-applying mechanism 208 may apply a tensionto the optical fiber 100 of less than or equal to about 90 g_(f), suchas less than or equal to about 85 g_(f). By minimizing the tension ofthe optical fiber 100, mechanical stresses formed in the optical fiberare reduced.

Example

Embodiments will be further clarified by the following example.

Five fibers, Samples 1-5, are formed comprising a germania-doped corehaving a Δ_(1MAX) of 0.256%. The five fibers have a fluorine-doped innercladding having a Δ_(2MIN)−0.094%. The fibers of Samples 2-5 have a puresilica glass outer cladding. The fibers are drawn at a tension of 100g_(f).

FIG. 3 graphically depicts Δ % versus radial position for fibers ofSamples 1-5. As shown in FIG. 3, the fiber of Sample 1 has a silica coredoped with GeO₂ to a radius of about 5 microns and a silica innercladding doped with F from a radius of about 5 microns to a radius ofabout 62.5 microns; the fiber of Sample 2 has a silica core doped withGeO₂ to a radius of about 5 microns, a silica inner cladding doped withF from a radius of about 5 microns to a radius of 50 microns, and anouter cladding of pure silica glass from a radius of 50 microns to aradius of 62.5 microns; the fiber of Sample 3 has a silica core dopedwith GeO₂ to a radius of about 5 microns, a silica inner cladding dopedwith F from a radius of about 5 microns to a radius of 45 microns, andan outer cladding of pure silica glass from a radius of 45 microns to aradius of 62.5 microns; the fiber of Sample 4 has a silica core dopedwith GeO₂ to a radius of about 5 microns, a silica inner cladding dopedwith F from a radius of about 5 microns to a radius of 40 microns, andan outer cladding of pure silica glass from a radius of 40 microns to aradius of 62.5 microns; and the fiber of Sample 5 has a silica coredoped with GeO₂ to a radius of about 5 microns, a silica inner claddingdoped with F from a radius of about 5 microns to a radius of 35 microns,and an outer cladding of pure silica glass from a radius of 35 micronsto a radius of 62.5 microns.

FIG. 4 graphically depicts axial stress versus radial position forfibers of Samples 1-5. As shown in FIG. 4, the fiber of Sample 1 has anaxial stress of about 26 MPa in its core; the fiber of Sample 2 has anaxial stress of about 17 MPa in its core; the fiber of Sample 3 has anaxial stress of about 14 MPa in its core; the fiber of Sample 4 has anaxial stress of about 12 MPa in its core; and the fiber of Sample 5 hasan axial stress of about 10 MPa in its core. Thus, the Example showsthat including an outer cladding that surrounds the inner claddingreduces the axial stress on the core of the optical fiber. Further, thecloser radially the outer cladding is to the core, the lower the axialstress in the core. However, as disclosed above, the outer cladding maybe positioned far enough away from the core so as not to interfere withthe light traveling through the optical fiber.

Table 1 shows examples of optical fibers comprising GeO₂-doped silicacore, fluorine (F)-doped silica inner cladding, and F-doped or undopedsilica outer cladding. Table 1 shows the core index, Δ_(max) in %(relative to pure silica having an index of 0.00% delta), the coredopant, the inner clad index Δ_(2max) in %, the inner clad dopant, theabsolute difference between the core and inner clad index in %, the coresoftening point (the softening point is defined as the temperature ofthe glass having a Log 10(viscosity)=7.6), the inner clad softeningpoint, the absolute difference in the core softening point and the innerclad softening point, the weight % GeO₂ dopant in the core[GeO_(2(core))], the weight % fluorine dopant in the inner clad[F_((iclad))] and the ratio (in weight %/weight %) of GeO₂ dopant in thecore to fluorine dopant in the inner clad [GeO_(2(core))/F_((iclad))]for Examples 1-37.

TABLE 1 Difference Core Inner in Softening index Inner clad Inner |Δ1max − Core Clad Point GeO2(core)/ (Δ1 max), Core index clad Δ2 min|,Softening Softening (Core − Inner GeO2(core), F(iclad), F(clad), Example% dopant (Δ2 min), % dopant % Point, ° C. Point, ° C. clad), ° C. wt. %wt. % wt. %/wt. % Ex 1 0.09 GeO₂ −0.11 F 0.20 1666 1648 18 1.55 0.37 4.2Ex 2 0.11 GeO₂ −0.09 F 0.20 1664 1652 12 1.91 0.30 6.3 Ex 3 0.13 GeO₂−0.07 F 0.20 1662 1656 6 2.27 0.24 9.5 Ex 4 0.15 GeO₂ −0.05 F 0.20 16611661 0 2.63 0.17 15.2 Ex 5 0.17 GeO₂ −0.03 F 0.20 1659 1665 −6 2.99 0.1127.4 Ex 6 0.12 GeO₂ −0.13 F 0.25 1663 1645 18 2.21 0.41 5.4 Ex 7 0.14GeO₂ −0.11 F 0.25 1661 1649 12 2.57 0.35 7.4 Ex 8 0.16 GeO₂ −0.09 F 0.251659 1653 6 2.93 0.28 10.4 Ex 9 0.18 GeO₂ −0.07 F 0.25 1658 1658 0 3.290.22 15.2 Ex 10 0.20 GeO₂ −0.05 F 0.25 1656 1662 −6 3.65 0.15 23.9 Ex 110.22 GeO₂ −0.03 F 0.25 1655 1667 −12 4.01 0.09 45.6 Ex 12 0.24 GeO₂−0.01 F 0.25 1653 1671 −18 4.37 0.02 186.8 Ex 13 0.16 GeO₂ −0.14 F 0.301660 1642 18 2.87 0.45 6.3 Ex 14 0.18 GeO₂ −0.12 F 0.30 1658 1646 123.23 0.39 8.3 Ex 15 0.20 GeO₂ −0.10 F 0.30 1656 1650 6 3.59 0.32 11.0 Ex16 0.22 GeO₂ −0.08 F 0.30 1655 1655 0 3.95 0.26 15.2 Ex 17 0.24 GeO₂−0.06 F 0.30 1653 1659 −6 4.31 0.20 22.0 Ex 18 0.26 GeO₂ −0.04 F 0.301652 1664 −12 4.67 0.13 35.6 Ex 19 0.28 GeO₂ −0.02 F 0.30 1650 1668 −185.03 0.07 75.3 Ex 20 0.20 GeO₂ −0.15 F 0.35 1657 1639 18 3.53 0.50 7.1Ex 21 0.22 GeO₂ −0.13 F 0.35 1655 1643 12 3.89 0.43 9.0 Ex 22 0.24 GeO₂−0.11 F 0.35 1654 1648 6 4.25 0.37 11.5 Ex 23 0.26 GeO₂ −0.09 F 0.351652 1652 0 4.61 0.30 15.2 Ex 24 0.28 GeO₂ −0.07 F 0.35 1650 1656 −64.97 0.24 20.8 Ex 25 0.30 GeO₂ −0.05 F 0.35 1649 1661 −12 5.33 0.17 30.5Ex 26 0.32 GeO₂ −0.03 F 0.35 1647 1665 −18 5.69 0.11 51.6 Ex 27 0.23GeO₂ −0.17 F 0.40 1654 1636 18 4.18 0.54 7.7 Ex 28 0.25 GeO₂ −0.15 F0.40 1652 1640 12 4.54 0.48 9.5 Ex 29 0.27 GeO₂ −0.13 F 0.40 1651 1645 64.90 0.41 11.9 Ex 30 0.29 GeO₂ −0.11 F 0.40 1649 1649 0 5.26 0.35 15.2Ex 31 0.31 GeO₂ −0.09 F 0.40 1647 1653 −6 5.62 0.28 19.9 Ex 32 0.33 GeO₂−0.07 F 0.40 1646 1658 −12 5.98 0.22 27.4 Ex 33 0.35 GeO₂ −0.05 F 0.401644 1662 −18 6.34 0.15 41.3 Ex 34 0.27 GeO₂ −0.18 F 0.45 1651 1633 184.84 0.58 8.3 Ex 35 0.30 GeO₂ −0.15 F 0.45 1649 1640 9 5.38 0.49 11.0 Ex36 0.33 GeO₂ −0.12 F 0.45 1646 1646 0 5.92 0.39 15.2 Ex 37 0.36 GeO₂−0.09 F 0.45 1644 1653 −9 6.46 0.29 22.0

The optical fibers in Table 1 have GeO₂-doped silica cores,fluorine-doped inner cladding, and fluorine-doped or undoped silicaouter cladding. The fibers in the Table are shown to have0.09≦Δ_(1max)≦0.36, and −0.18≦Δ_(2min)−0.01. The fibers in the Table areshown to have 0.20≦|Δ_(1max)−Δ_(2min)|≦0.45. The fibers in the Table areshown to have an absolute difference in the core softening point and theinner clad softening point of ≦20° C., in some examples ≦15° C., and insome examples ≦10° C. The fibers in the Table are shown to have a weight% of GeO₂ dopant in the core of ≦6.5%. The fibers in the Table are shownto have 1.5 wt. %≦GeO_(2(core))≦6.5 wt. %. The fibers in the Table areshown to have ≧0.02 wt. % fluorine dopant in the inner clad. The fibersin the Table are shown to have 0.02 wt. %≦F_((iclad))≦0.6 wt. %. Thefibers in the Table are shown to have a ratio of GeO₂ dopant in the coreto fluorine dopant in the inner clad (in weight %/weight %) of4<[GeO_(2(core))/F_((iclad)))]<190, in some embodiments6<[GeO_(2(core))/F_((iclad))]<50, in some embodiments6<[GeO_(2(core))/F_((iclad))]<35.

Table 2 shows the core, inner cladding and outer cladding indexes(relative to pure silica having an index of 0.00% delta), the core outerradius, alpha and dopant, the inner clad outer radius and dopant, andthe outer clad radius and dopant of exemplary fibers according toembodiments disclosed herein. Also shown are the optical properties forthese fibers including the zero dispersion wavelength (lambda 0), the1310 and 1550 nm dispersion, dispersion slope, and mode field diameter,theoretical and 22 meter cable cutoff, 1550 nm effective area, lateralload and pin array and attenuation at 1550 nm (“na” equals notapplicable). In Table 2: Example 38 corresponds to Example 4; Examples39 and 40 correspond to Example 9; Example 41 corresponds to Example 16;Examples 42 and 43 correspond to Example 23; Example 44 corresponds toExample 30; and Example 45 corresponds to Example 36.

TABLE 2 Ex. 38 Ex. 39 Ex. 40 Ex. 41 Ex. 42 Ex. 43 Ex. 44 Ex. 45 Coredelta, 0.15 0.18 0.18 0.22 0.26 0.26 0.29 0.33 Δ_(1max), % Core radius,R₁, 6.60 6.14 6.14 5.41 4.36 4.36 4.16 3.96 microns Core alpha 20 20 2020 20 20 20 20 Core dopant GeO₂ GeO₂ GeO₂ GeO₂ GeO₂ GeO₂ GeO₂ GeO₂ Innerclad delta, −0.05 −0.07 −0.07 −0.08 −0.09 −0.09 −0.11 −0.12 Δ_(2min), %Inner clad radial 15 15 50 15 15 55 15 15 thickness, microns Inner claddopant F F F F F F F F Outer clad delta, −0.05 −0.07 0.00 −0.08 −0.090.00 −0.11 −0.12 Δ₃, % Outer clad radial 62.5 62.5 62.5 62.5 62.5 62.562.5 62.5 thickness, microns Outer clad dopant F F none F F none F FDispersion at na na na na 0.80 0.80 0.74 0.54 1310 nm, ps/nm/kmDispersion slope na na na na 0.085 0.085 0.085 0.084 at 1310 nm,ps/nm/km/nm Lambda 0, nm na na na na 1301 1301 1301 1304 Dispersion at20.5 20.5 20.5 19.8 17.4 17.4 17.1 16.7 1550 nm, ps/nm/km Dispersionslope 0.061 0.061 0.061 0.060 0.057 0.057 0.056 0.056 at 1550 nm,ps/nm/km/nm Mode Field na na na na 9.1 9.1 8.5 8.1 Diameter at 1310 nm,microns Effective Area at 156 129 129 104 80.7 80.7 73 63.9 1550 nm,microns² Mode Field 14 12.7 12.7 11.5 10.2 10.2 9.7 9.1 Diameter at 1550nm, microns Pin Array at 55 14 14 5.1 5.5 5.5 1.5 0.4 1550 nm, dBTheoretical 1549 1611 1611 1556 1354 1354 1382 1396 Cutoff, nm 22 mcable 1299 1461 1461 1406 1204 1204 1232 1246 cutoff nm Lateral Load at7.5 1.6 1.6 0.51 0.19 0.19 0.11 0.08 1550 nm, dB Attenuation at ≦0.18≦0.18 ≦0.18 ≦0.18 ≦0.19 ≦0.18 ≦0.19 ≦0.19 1550 nm, dB/km

The optical fibers in Table 2 have GeO₂-doped silica core andfluorine-doped inner cladding, and fluorine-doped or undoped silicaouter cladding. The fibers in Table 2 are shown to have0.15≦Δ_(1max)≦0.33, and −0.12≦Δ_(2min)≦−0.05. The fibers in Table 2 areshown to have 0.20≦|Δ_(1max)−Δ_(2min)|≦0.45. The fibers in Table 2 areshown to have 1310 mode field diameters between about 8.1 and 9.4microns² and Lambda 0 between 1300 and 1324 nm and cable cutoff of lessthan or equal to 1260 nm. The fibers in Table 2 are shown to have 1550mode field diameters of between about 9 to 14 microns² and cable cutoffof less than 1500 nm. A number of examples of fibers in Table 2 areG.652 and G.654 compliant. The fibers in Table 2 are shown to have lowattenuation of ≦0.19 dB/km.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the embodiments describedherein without departing from the spirit and scope of the claimedsubject matter. Thus it is intended that the specification cover themodifications and variations of the various embodiments described hereinprovided such modification and variations come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A single mode optical fiber comprising: a corecomprising silica and greater than or equal to 2 weight % and less thanor equal to 6.5 weight % germania, and having a maximum relativerefractive index Δ_(1MAX); a fluorine doped inner cladding surroundingthe core and having a minimum relative refractive index Δ_(2MIN),wherein the core and the inner cladding having compositions such that adifference between a softening point of the core and a softening pointof the inner cladding is less than or equal to 20° C., and whereΔ_(1MAX)>Δ_(2MIN).
 2. The single mode optical fiber of claim 1, whereina difference between a softening point of the core and a softening pointof the inner cladding is less than or equal to 15° C.
 3. The single modeoptical fiber of claim 1, wherein the inner cladding comprises fromgreater than or equal to 0.1 weight % fluorine to less than or equal to0.65 weight % fluorine.
 4. The single mode optical fiber of any of thepreceding claims, wherein the core is doped with GeO₂, the innercladding is doped with fluorine, and6<GeO_(2(core))/fluorine_((iclad))<35, where GeO₂ and F are in weight %.5. The single mode optical fiber of claim 1, wherein the core has amaximum relative refractive index, Δ_(1MAX), from greater than or equalto 0.13% to less than or equal to 0.37%.
 6. The single mode opticalfiber of claim 1, wherein the inner cladding has a minimum relativerefractive index, Δ_(2MIN), from less than or equal to −0.04% to greaterthan or equal to about −0.21%.
 7. The single mode optical fiber of claim1, wherein the core has a radial thickness from greater than or equal to3 microns to less than or equal to 7 microns.
 8. The single mode opticalfiber of claim 1, wherein the inner cladding has a radial thickness fromgreater than or equal to 12 microns.
 9. The single mode optical fiber ofclaim 1, further comprising an outer cladding surrounding the innercladding, wherein the outer cladding consists essentially of silica orSiON, the outer cladding has a maximum relative refractive indexΔ_(3MAX), and Δ_(3MAX)>Δ_(2MIN).
 10. The single mode optical fiber ofclaim 9, wherein the inner cladding has a radial thickness from greaterthan or equal to 12 microns to less than or equal to 55 microns.
 11. Thesingle mode optical fiber of claim 1, wherein the single mode opticalfiber has an attenuation of less than or equal to 0.18 dB/km at awavelength of 1550 nm.
 12. The single mode optical fiber of claim 1,wherein the single mode optical fiber has an effective area at 1550 nmof greater than or equal to 70 microns².